1. Technical Field
The present invention relates to a medium transport device which reverses an upside and a downside of a medium such as paper transported by a transport roller with a first face facing toward one direction, by switching of forward rotation and reverse rotation of a discharge roller, to transport the medium with a second face of the medium facing toward the one direction, and more particularly, to a medium transport device and an image processing apparatus provided with the medium transport device to perform switching of a nip state and a release state of the discharge roller at the optimal time irrespective of differences in the lengths of the mediums.
2. Related Art
As described in JP-A-2007-230657, an image reading apparatus such as a photocopier, a facsimile, and a scanner provided with an auto document feeder as an example of a medium transport device has been already developed. Such an image reading apparatus is provided with an image reading device capable of continuously reading image information printed on both front and back faces of a medium (hereinafter, referred to as “paper”), and the image reading device guides paper fed with the first face facing upward, to a semi-loop-shaped transport path on which a transport roller and a discharge roller are provided, to read an image of a first face with the first face facing downward.
Then, the paper is allowed to enter a reverse path with the transport direction switched, and the paper is transported with a second face, which is opposite to the first face, facing downward by guiding it to the transport path again to read an image of the second face.
The switching of the transport direction of the paper and the switching of the nip state and the release state of the discharge roller, which are necessary to perform continuous image reading of the first face and the second face of the paper, are performed in a fixed operation time preset by a medium transport device (hereinafter, referred to as “paper transport device”).
The paper transport path gets smaller and shorter as the image reading apparatus gets smaller. Accordingly, when paper with a long paper length is transported, missing occurs between a paper sent to the reverse path and a paper discharged from the transport path to the outside, at the nip position of the discharge roller at the second face transport time. Thus, the discharge roller is allowed to be in the release state when the missing occurs, to allow the missing to occur.
However, when the switching of the transport direction of paper and the switching of the nip state and the release state of the discharge roller are performed by one driving motor, such an operation time becomes a fixed time determined by a deceleration ratio of gear rows and the like. Accordingly, the length of paper on which the first face transport and the second face transport are continuously performed is limited to a fixed range (for example, A4 length size to letter length size). Therefore, conventional paper transport devices cannot cope with paper with a length in a wide range in which the length of paper is, for example, A4 width size to a size of 17 inch (210 mm to 432 mm) since there is also the problem of the missing.
The switching of the nip state and the release state of the discharge roller can be performed by separate power separated from the power of the transport roller or the discharge roller, for example, a solenoid disclosed in JP-A-2004-2024. However, the number of components is increased or the structure becomes complicated, thereby raising costs of the paper transport device.